Metal Coated Substrates for Filters

ABSTRACT

An anti-microbial metal coating may be applied to filter membranes for use in actively depressing microbial viability in filtration applications. The anti-microbial metal coating may be applied to substrates that are considered to be sensitive to damage by conventional metal coating techniques or resistant to metal bonding. The coating may be applied from a salt absorbed to the substrate in solution, converted to a reducible form with a conversion agent, and reduced to active metal format through a low temperature plasma treatment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. application Ser. No.17/209,970, filed on Mar. 23, 2021, and entitled “Anti-Microbial MetalCoatings for Filters”, the content of which being incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to substrate coatings generally, and moreparticularly methods for obtaining metal coatings on substrates foranti-microbial and reactive electrochemical filtration and otherapplications. The methods of the present invention may be particularlysuitable for porous substrates that are susceptible to damage whenprocessed through conventional metallization techniques.

BACKGROUND OF THE INVENTION

Metals are known for their anti-microbial properties. Copper and silverhave long been used to purify water and to preserve perishable fooditems. More recently, titanium, platinum, palladium, zinc, and othermetals have also been explored for their anti-microbial properties.Copper and silver remain popular as anti-microbial agents due to theirrelatively low cost and high bioavailability.

Copper and copper alloys have gained renewed importance in recent yearsfor use in neutralizing viruses that are harmful to the humanpopulation, including the coronavirus that causes COVID-19. To date,however, no effective technique has been developed to incorporate metalssuch as copper in personal protection equipment (PPE) or filters for airor water disinfection, although it is well known to coat metal withpolymeric materials to improve performance, corrosion resistance andlongevity. Filter membranes prepared by incorporating pulverized metalor metal salts in polymer matrices, for example in polymer solutions ormelts before spinning into fibers have found limited success because alarge percentage of the metal remains embedded in the bulk of thepolymer.

Sputtering, physical vapor deposition (PVD), and chemical vapordeposition (CVD) have been widely used for depositing metal films ontovarious substrates. Each technique, however, has challenges for use inconnection with microporous plastic substrates. Physical vapordeposition typically requires extremely low pressure environments and/orexpensive coating equipment, since the metals need to be vaporized.Chemical vapor deposition and sputtering techniques may be performed atsomewhat higher pressure environments, but are more suited for coatingextremely thin films of silicon, gallium, arsenide, and the like onsilicon wafers used in the semiconductor industry. When used on porousplastic substrates, textures and residual stresses may arise insputtered metal layers that can result in the deformation of thesensitive membrane structure. Moreover, CVD and sputtering techniquesare not generally suited for continuous application, such as inproducing metal-coated HEPA filters.

U.S. Pat. Nos. 4,252,838 and 4,717,587 provide a method of producingmetallic structures on nonconductive substrates by glow dischargedisintegration of volatile organometallic complexes for semiconductorapplications. The technique, however, is useful only for depositingmetals that form volatile complexes. Such complexes are not onlyexpensive but also produce a very low yield of metal as the majority ofthe complex is lost unconverted in the vacuum process.

U.S. Pat. No. 5,516,458 describes a method for depositing an antistaticmetal containing transparent coating on thermal imaging films byapplying a source of metal mixed in a polymeric film former liquidcomposition followed by glow discharge treatment at very low pressure of10⁻⁵ mbar. The resultant coating had surface resistivities ranging from10⁸-10¹³ ohms/□. Such a technique will not be useful for preparingantimicrobial metal coating on microporous substrates as the polymericfilm former will block the pores of the substrate.

U.S. Pat. No. 7,258,899 describes a method for coating metals onmicroporous substrates based on polypropylene, polyvinylidene fluoride,polytetrafluoroethylene, ceramics, and the like by applying a coating oforganometallic complex of metal dissolved in organic solvents such astoluene, xylene, acetone, and alcohol by dip coating, followed byreduction of the organometallic complex to metal by low pressure plasma.The method, however, suffers from the disadvantage that manyorganometallic complexes are highly volatile and not suited for plasmareduction, which is often performed at low pressures. Many others arenot readily soluble in organic solvents.

U.S. Pat. No. 7,666,494 B2 provides a method for producing metallicnanoparticles on a microporous substrate that are subsequentlytransferred to other substrates such as glass, plastic and metals forimparting special optical, magnetic and chemical properties forapplication in infrared detectors, sensors and optical switchingdevices. The metal nanoparticles are created by physical vapordeposition such as electron beam evaporation or sputtering. The processoperates at ultrahigh vacuum and the deposited nanoparticles are onlyphysically held in the microporous substrate which acts as a storagevessel for the metal.

Attempts have been made to apply metal salts to substrates followed byreduction of the salt to metal by strong chemical reducing agents suchas ascorbic acid or sodium borohydride, which can result in substantialreduction in the tensile strength of the substrates. Another recentlyreported method involves the application of a composite metallic inkcomprising a metal salt, a binder, and a reducing agent to a microporoussubstrate by dip coating. The coated substrate is heated to convert themetal salt into a sintered metal coating. This method, however, usestoxic chemicals, requires heat, has the tendency to block substratepores, and is therefore not suited for sensitive membrane substrates.

There is therefore a need for an efficient and inexpensive process forcoating cellulosic and non-cellulosic polymers with active metal speciesthat possess effective anti-microbial properties.

SUMMARY OF THE INVENTION

By means of the present invention, metal salts, both organic andinorganic, may be deposited on a porous substrate surface and chemicallyconverted to insoluble oxides or hydroxides while at the substrate, andreduced to an active metal form without significant change in theporosity or tensile strength of the substrate. The reduction may beperformed by a low-temperature plasma process in a continuous,semi-continuous, or batch operation. Through this process, the metalactivity is primarily retained at the substrate surface, and typicallyin a nanoparticle form. The anti-microbial metal coatings of the presentinvention may find application in PPE, air and water filtrationmembranes, and in wound healing dressings. These metal coated membranescan also find application in reaction catalysis, in bioreactors forwaste water treatment, microbial fuel cells, biosensors, electroceuticalfabrics, electrochemical processes, antifouling area waste filters,solar cells, supercapacitors, batteries, membrane distillation,evaporative membrane applications, and in the design of smart textiles.

In an embodiment of the invention, a metal is coated onto a substrate byapplying a metal solution to the substrate, wherein the metal solutionincludes a solvent and a metal component selected from one or moreinorganic or organic metal salts that are soluble in the solvent. Themetal component solution is reacted at the substrate with a conversionagent to form an insoluble metal hydroxide or a metal oxide, which isthen exposed to a plasma environment at ambient temperature and lessthan 100 KPa pressure to reduce the metal hydroxide or metal oxide to ametal coating at the substrate.

In an embodiment wherein the metal component solution includes water asthe solvent, the metal component may include one or more water solublemetal salts. The one or more water soluble metal salts may be selectedfrom at least one of a sulfate, nitrate, nitrite, carbonate,bicarbonate, chloride, chlorate, arsenate, phosphate, formate, acetate,and propiontae. The substrate in this embodiment may be cellulose-based,ceramic, or porous substrates such as Polybenzimidazole orpolyethersulfone.

In an embodiment wherein the metal component solution includes anorganic solvent, the metal component may include an organometalliccomplex of metal, optionally with oxygen linkage such as anacetylacetonoate, a perfluoroacetate, or a fluoroacetylacetonate. Insome embodiments, the organometallic complex of metal is selected fromat least one of copper acetylacetonate, copper trifluoro acetylacetonateand silver trifluoroacetate. In preferred embodiments, the metal complexis converted to a hydroxide or oxide which is stable and reducible tometal under plasma conditions.

In an embodiment, the metal component solution may be applied to thesubstrate by spray coating, roller coating, brush coating, or dipcoating.

In an embodiment, the conversion agent is selected from at least one ofa hydroxide of alkali metal.

In an embodiment, the reactive metal layer on the substrate may beheated by applying a low electric voltage which can further deactivatemicrobes, regenerate the substrate, and accelerate the rate of physical,chemical or biochemical reaction.

In an embodiment, the plasma environment is generated by applyingelectromagnetic energy in a glow zone between spaced apart electrodes inthe presence of a plasma supporting gas or gas blends selected fromargon, hydrogen, krypton, xenon, helium, nitrogen, and oxygen. Theelectromagnetic energy may be applied at 13.56 MHz.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An object of the invention is to provide metallic coatings on poroussubstrates from simple inorganic and organic metal components throughlow-temperature plasma conversion of their hydroxides and oxides tometal. Solutions containing metal components are applied to thesubstrates, with the metal components being chemically converted with asuitable conversion agent to the metal hydroxides or metal oxides.Exposure of the metal hydroxides or metal oxides to ambient temperatureplasma under mild operating conditions produce active metal coatingsuseful for many applications including anti-microbial products.

A particular object of the invention is to provide metallic coatings onporous substrates that are susceptible to damage from conventional metalcoating techniques. Example substrates suitable for the metallic coatingprocess of the present invention include materials such as regeneratedbase filter paper, glass filters, personal protection equipment based oncotton fabric, nonwoven cotton dressings, water filtration candles, andmembranes based on ceramics and other water wettable polymers such aspolyethersulfone (PES) and polybenzimidazole, as well as filtermembranes based on other polymers such as polypropylene, polyethylene,polyvinylidene fluoride, and nylon.

Use of active plasma for reducing adsorbed metal complexes to thin filmsof metals has been demonstrated by the present inventors in U.S. Pat.No. 6,136,389, as well as in U.S. Pat. No. 7,258,899, the contents ofwhich being incorporated herein by reference in their entireties.

When coated onto fibers, tubular membranes, or flat substrates, coatingscreated pursuant to the present invention may serve in a variety ofapplications, as described above.

The plasma treatment of the coated substrate may be achieved using aknown plasma reactor. A capacitively coupled tubular reactor operatingat 13.56 MHz may be employed in the plasma reductions described herein.Custom-designed reactors may be utilized if required for continuouscoating of substrates such as hollow fiber and films. One such reactoris described in U.S. Pat. No. 4,824,444.

The chosen substrate may be contacted with the metal component solutionby any suitable means such as spray coating, brush coating, dip coating,roller coating, sponge coating, and the like. The selected substrate ispreferably wettable by the metal component solution, wherein the surfacetension of the metal component solution is lower than the surface energyof the substrate, so that the liquid is able to maintain contact withthe substrate. In the case of substrates which do not readily wet uponcontact with the selected metal component solution, chemical andphysical methods including a preliminary exposure to a suitable plasmasurface treatment may be used to enhance the wettability andspreadability of the metal component solution onto the surface and/orinto the substrate pores.

The metal component solution includes a solvent and a metal componentselected from one or more metal salts or one or more organometalliccomplexes of metal. Example water-soluble salts include sulfates,nitrates, nitrites, arsenates, arsenites, chlorides, chlorates,formates, acetates, propionates, carbonates, and bicarbonates. Aqueoussolutions of water-soluble metal salts may be preferred for applicationto polar substrates, such as cellulose-based substrates. It iscontemplated that one or more metal combinations, alloys, mixtures, andmetal complexes may be employed in the metal coatings of the presentinvention. Example water-soluble salts useful in the present inventioninclude inorganic salts of copper, silver, cobalt, tin, zinc, palladium,and nickel. Organometallic complexes and organic salts of metals may beused in organic solvents such as methanol, ethanol, or other suitablesolvents to form the metal component solution. Example organicsolvent-soluble organic salts of metal include silver trifluoroacetate,and organometallic complexes of metals include copperacetylacetonate,copper trifluoroacetylacetonate and silver trifluoroacetyacetonate.

The metal component solution at the substrate is preferably reacted witha conversion agent to form a metal hydroxide or a metal oxide. Theconversion reaction may be conducted in “wet” conditions, prior tosolvent evaporation, or in the “dry” condition after solventevaporation. The conversion agent is preferably a chemical that isreactable with the metal component solution to form a reducible waterinsoluble metal compound. In some embodiments, the conversion agent is achemical that is reactable with the metal solution to form a metalhydroxide or oxide. Example conversion agents include hydroxides ofalkali metals such as sodium hydroxide and potassium hydroxide.

Subsequent to the conversion reaction, the system may be washed free ofexcess conversion agent and dried, for example in an air-forced oven orin ambient conditions to evaporate the solvents, leaving the reduciblemetal compounds at the substrate surface.

The substrates are subsequently mounted in a plasma reactor, and thesystem evacuated to a pressure of less than 100 KPa. Example plasmatreatment conditions include a plasma gas selected from argon, hydrogen,oxygen, blends of oxygen and nitrogen, blends of hydrogen and nitrogen,krypton, xenon, helium, and other plasma-excitable gases. The gas flowrate may be between 1-500 SCCM, preferably between 5-100 SCCM at asystem pressure of less than 100 KPa, preferably less than 10 KPa, andmore preferably between 0.001 KPa and 1 KPa. In some embodiments, thesystem pressure is between 0.01-0.1 KPa. The discharge power of theplasma system may be between 1-500 watts, and preferably between 5-150watts, at 13.56 MHz for 15 seconds to 30 minutes.

The reduction to active metal at the substrate surface may begin nearlyinstantaneously upon exposure to the plasma environment. The plasmaconditions described above are exemplary only, and may change dependingupon the size of the reactor, substrate material, power, source, andplasma coupling mechanisms.

The substrates may be rotated during the plasma exposure for betteruniformity and continuous web or fiber strands may be moved continuouslythrough the plasma. After plasma treatment, vacuum is released usingstandard venting techniques.

The substrates may be re-exposed as well as re-coated and re-processedif additional metal coating is desirous or if initial coating wasincomplete. Multiple layered coatings may also be readily accomplishedby re-processing the substrates.

Alloys may be formed by suitable mixing of multiple metal componentmaterials in compatible solvents. Deposition of the metal coatings maybe manipulated for placement and coverage on the substrate, includingthough the use of masking techniques.

The metal coatings of the present invention may be consideredconductive, and which exhibit a relatively low surface resistivity. Thelow surface resistivity may be facilitated by a continuous metal layerdeposited on the substrate. In some embodiments, the continuous metallayer may be formed at the substrate so as to not interfere with theporosity of the substrate, wherein pores of the substrate are openthrough the metal coating. This is possible with a thin metal coatingthat is deposited on the substrate surface surrounding the pore opening,but does not cover the pore opening itself.

Certain applications of the metal coated substrates of the presentinvention may be benefited by a surface that is not electricallyinsulative. A “conductive” material is considered to have a surfaceresistivity of less than 1×10⁵Ω/□. A “dissipative” material isconsidered to have a surface resistivity of between 1×10⁵Ω/□ and1×10¹²Ω/□. An “insulative” material is considered to have a surfaceresistivity of at least 1×10¹²Ω/□. Thus, in some embodiments, the metalcoated substrate may preferably exhibit a surface resistivity of lessthan 1×10¹²Ω/□. In some embodiments, the metal coated substrate maypreferably exhibit a surface resistivity of less than 1×10⁵Ω/□. In stillfurther embodiments, the metal coated substrate may preferably exhibit asurface resistivity of less than 1×10³Ω/□. In some embodiments the metalcoated substrate may preferably exhibit a surface resistivity of lessthan 1×10²Ω/□.

EXAMPLES

The following specific examples are provided as demonstrative of thetechniques of the present invention.

Example 1

Copper sulfate pentahydrate (CSP) is soluble in water and can be easilyabsorbed on cellulosic substrates. The absorbed CSP can be hydrolyzed bysodium hydroxide to copper hydroxide, which can then be reduced tocopper metal using the plasma treatment process. The copper metalremains absorbed in the fibril structure of the cellulosic substrate,and does not leach out easily in water.

Four drops of 25% w/v CSP solution in water were applied to Whatman No.5 filter paper and allowed to dry in air for 12 hours followed byreaction with 1.5N sodium hydroxide (NaOH) solution in a petri dish forone minute. The color of the drop impressions immediately changed fromalmost colorless to blue indicating conversion to copper hydroxide. Dueto its very poor solubility in water, the copper hydroxide remainedbonded to the filter paper, resulting in almost 100% conversion. Thecopper hydroxide coated filter paper was subsequently washed with waterto get rid of free chemicals, and then dried in air at ambienttemperature for 12 hours to offer a copper hydroxide coated substratewhich was insensitive to water.

Example 2

The copper hydroxide coated Whatman #5 filter paper of Example 1 wastreated with low pressure Argon (Ar) plasma generated at 40 mtorr for 3minutes at 20 watts RF power. The conversion to metal started almostinstantaneously. The converted metal coating exhibited a surfaceresistivity of 195Ω/□ (Ohms/square).

Example 3

The copper hydroxide coated substrate prepared in Example 1 above wastreated with low pressure Ar/H₂ (Argon/Hydrogen) plasma for 6 minutes at40 mtorr pressure and 20 watts RF power. The resultant metal coatingexhibited a surface resistivity of 25Ω/□ and showed substantialanti-microbial property toward the E Coli K-12 strain (see the testmethod described in Example 17 and results in Table 1).

Example 4

A solution of 25% w/v of copper sulfate pentahydrate (CSP) in water wasapplied to a 4″×4″ piece of ‘hydro entangled Cellulose-Polyester’ cleanroom wipe cloth (source: Amplitude Sigma) by immersing the wipe cloth inthe solution. The soaked substrate was then immersed in 1.5N NaOHsolution for approximately 1 minute without allowing for drying inbetween the two treatments. The conversion to blue copper hydroxide wasinstantaneous. Once again due to very poor solubility of copperhydroxide in water, the converted salt remained bonded to the substrate,resulting in almost 100% conversion. The copper hydroxide coatedsubstrate was subsequently washed with copious amount of water todispose of free chemicals, and then dried in air at ambient temperaturefor 2 hours. The dried copper hydroxide coated substrate was treatedwith low pressure argon/hydrogen plasma generated at 13.56 MHz for 5minutes at 50 W. The copper hydroxide coating was reduced to a darkbrown coating of metallic copper. The metal-coated substrate exhibited asurface resistivity of 110Ω/□ and showed substantial anti-microbialactivity toward the E Coli K-12 strain (see results in Table 1).

Example 5

A 3″×3″ piece of cotton bandage (Johnson and Johnson) was coated with25% w/v solution of CSP and converted to copper hydroxide as describedin Example 4. The copper hydroxide was reduced to copper metal throughthe plasma treatment described as in the Example 4. The plasma treatedcotton bandage exhibited a surface resistivity of 400Ω/□ from end toend, and showed substantial anti-microbial property toward the E ColiK-12 strain.

Example 6

A 10% w/v solution of silver nitrate in water was prepared and appliedonto Whatman filter paper #5 by a dropper. The solution drops wereallowed to dry for approximately 15 minutes in air and subsequentlytreated with 1.5 N sodium hydroxide solution in a petri dish for 30 sec.The colorless drops of silver nitrate immediately changed in color tobrown indicating chemical conversion to Silver oxide. Argon plasmatreatment of silver oxide at 125 mtorr, and 25 W RF power for 3 minuteslead to the formation of silvery coatings on the filter paper whichexhibited a surface resistivity of 25Ω/□. Increasing the treatment timeto 6 minutes lead to silver coating which exhibited a surfaceresistivity of 5Ω/□. The coating remained stable in water for 4 days andshowed significant antimicrobial activity to E. Coli K-12 strain (seeresults in Table 1).

Example 7

A 10% solution of silver trifluoroacetate in ethyl alcohol solvent wasapplied to a cellulosic substrate. The dried clear coating of silvertrifluoroacetate was treated with Ar/H2 plasma at a pressure of 200mtorr as in the earlier experiment which converted the silvertrifluoroacetate to a dark brown coating, but surprisingly this coatingwas not electrically conductive and exhibited a very high surfaceresistivity. It appears that the trifluoroacetyl moiety in the structureof salt interferes with the metallizing mechanism under plasma.

Example 8

A 10% solution of silver trifluoroacetate in ethyl alcohol solvent wasapplied on to a cellulosic substrate followed by reaction with 2.5N NaOHsolution, which converted the clear coating of silver trifluoroacetateinto dark brown silver oxide. The coating was thoroughly washed withwater and treated with Ar/H2 plasma at a pressure of 200 mtorr as in theearlier experiment which converted the oxide into a shiny conductivesilver that exhibited a surface resistivity of 5 Ω/□.

Example 9

A 10% solution of silver trifluoroacetate in ethyl alcohol solvent wasapplied on to microporous polypropylene and polyether sulfone hollowfiber membranes using a dip coating apparatus, followed by reaction with2.5N NaOH solution which instantaneously converted the silver salt tosilver oxide. The coated substrates were treated with argon and/orhydrogen plasma as described in Example 4, to render a bluish brownconductive metal coating. Both coated substrates retarded the growth ofthe E Coli K-12 strain, as shown in Table 1.

Example 10

A solution of 7.5% w/v copper trifluoroacetylacetonate (CuTFAA) inethanol was applied to Whatman Filter paper #5 in drop form. The coatedsubstrates, after a very short evacuation time of 2 minutes, was treatedwith argon/hydrogen plasma as described in Example 4 to render a blackmetal coating. The coated Whatman #4 filter paper exhibited a surfaceresistivity of 300-400Ω/□, and retarded growth of the E Coli K-12strain. This compound did not need conversion to hydroxide and or oxidebecause of labile metal-oxygen linkage in the structure. Longerevacuation time of 40 minutes (Sample 10A), however, led to loss of saltfrom the substrates due to low sublimation temperature of the coppercomplex and resulted in non-conductive, non-antimicrobial coatings.

Example 11

A solution of 7.5% w/v copper trifluoroacetylacetonate (CuTFAA) inethanol was applied to Whatman Filter paper #5 by a dropper and allowedto dry. The coated substrate was treated with 2.5N NaOH solution for 2minutes in a petri dish and after thorough washing in distilled waterallowed to dry. Unlike the original organometallic complex, theconverted salt was completely insoluble in alcohol. The dried hydrolyzedsalt, after a long evacuation time of 60 minutes, was treated with Ar/H2plasma as in the above experiments. The color of the coating immediatelychanged to black coating indicative of conversion to metal, which alsodemonstrated good antimicrobial activity. Conversion of volatile metalcomplex to nonvolatile hydroxide helped in improving its stability invacuum.

Example 12

A 15% w/v solution of cobalt chloride hexahydrate (CCH) in water wasprepared and applied onto Whatman filter paper #5 by a dropper. Thesolution drops were allowed to dry for approximately 15 minutes in air.The magenta color of CCH changed to deep blue on drying indicatingconversion to the anhydrous form of the salt. The dried salt drops weresubsequently treated with 1.5 N Sodium hydroxide solution in a petridish for 30 sec. The bluish drops of anhydrous Cobalt chlorideimmediately changed to olive color indicating chemical conversion tocobalt hydroxide. Argon/Hydrogen plasma treatment of cobalt hydroxide at125 mtorr, and 50 W RF power for 5 minutes lead to the formation ofbluish black cobalt coatings on the filter paper. The coated substrateshowed substantial anti-microbial property toward the E Coli K-12strain.

Example 13

Blends of CSP (25% w/v) and CCH (15% w/v) were prepared by mixing thesalt solutions in a 1:1 ratio. Strips of Whatman #5 filter paper werecoated with the blend solution by the dipping process followed by shortdrying for approximately 15 minutes in air. The coated substrate wasthen treated with 1.25 N NaOH for approximately 1 minute followed bythrough washing in running tap water. The hydrolyzed coating was driedin air and subsequently treated with Argon/Hydrogen plasma as in earlierexamples. The color of coating turned from pale bluish green to greenblack after metallization. Unlike the untreated salt the converted metalhydroxide and metal coatings were completely insoluble in water.

Example 14

A blend of CSP (25% w/v), CCH (15% w/v) and Nickel Chloride hexahydrate(NCH) (15% w/v) was prepared by mixing the aqueous salt solutions in1:1:1 ratio. Strips of Whatman #5 filter paper were coated with theblend solution by the dipping process followed by short drying forapproximately 15 minutes in air. The coated substrate was then treatedwith 1.25 N NaOH for approximately 1 minute followed by through washingin running tap water. The hydrolyzed coating was dried in air andsubsequently treated with Argon/Hydrogen plasma as in earlier examples.The color of coating turned from bluish green to brownish green aftermetallization. Unlike the salt, the converted metal hydroxide and metalcoatings were completely insoluble in water and exhibited substantialantimicrobial activity (see Table 1).

Example 15

Strips of Whatman #5 filter paper were coated with CSP solution ofvarying concentrations (25%, 10%, 5%, 2.5% and 1.0% w/v) by dippingmethod and dried in air. The dried salt on strips in each case washydrolyzed in 1N NaOH (samples a, b, c, d and e) for approximately 1minute. In addition, the strips coated with 25% w/v CSP solution werealso hydrolyzed in 1N KOH, 0.5 NaOH and 0.2 NaOH solution (sample f, gand h). All hydrolyzed samples were thoroughly washed with deionizedwater and allowed to dry in air. The dried samples were subsequentlytreated by hydrogen/argon plasma as in the Example 4. All samplesdarkened on exposure to plasma with the color intensity proportional tothe concentration of the CSP solution used. While samples a, b, f, g andh exhibited varying degree of surface resistivity as well as goodantimicrobial resistance, the lower salt concentration coated sampleexhibited no measurable conductivity with the meter employed. The lowermetal concentration apparently had non-continuous coating of extremelyhigh surface resistivity due to the large pore structures of thesubstrates. No significant difference was observed of using lowerconcentration of NaOH or KOH for hydrolysis.

Example 16

A cleaned 5 mm diameter CPG (controlled pore glass) tube, approx. 1 inchin length was dip coated with silver nitrate solution, 25% w/v in water,and allowed to dry in air. The coated CPG tube was treated with 1.25 NNaOH solution for converting silver nitrate salt to Silver oxide. Thesubstrate was washed in flowing tap water and dried in air. The Silveroxide coated CPG was subsequently treated with Argon/Hydrogen plasma at100 W for 3 minutes. The resultant metal coating offered a surfaceresistivity of 200 Ω/o, indicating that the salt had been converted tometal.

Example 17

Metal coatings prepared in some of the examples above were tested fortheir antimicrobial activity by placing them on a pre-poured LB agarplate inoculated with E. Coli (stain K12 cultured in TSB). The plateswere left in the culture oven at 35 deg C. for 16-18 hours. The width ofthe resulting zone of inhibition from the edge of sample to the edge ofbacterial colonies, was noted in each case. In a few instances, theexperiment was repeated with metal coatings kept immersed in water for96 hours to see the effect of coating exposure to water. The resultssummarized in table confirmed that the metal coating prepared by thismethod exhibited a zone of retarded bacterial growth and coatingsretained their efficacy even after exposure to water.

TABLE 1 Zone of Inhibition Metal Zone of after exposure Example# MetalConcentration Inhibition to water Example 3 Copper 25% 10 mm NA Example4 Copper 25% 4 mm NA Example 6 Silver 10% 6 mm 4 mm Example 9 silver 10%4 mm NA Example 10 Copper 7.5%  2 mm NA Example 10A Copper 7.5%  —Example 11 Copper 7.5%  1.5 mm NA Example 12 Cobalt 15% 2 mm NA Example14 Cobalt + 15% 1 mm NA Copper + Nickel Example 15 Copper 25% 2 mm 1 mmExample 15 Copper 10% 1 mm 1 mm

Example 18

A 12 ml vial was filled with the broth solution and inoculated with 50ul E Coli bacteria. Whatman filter strips coated with metals wereimmersed in the broth and allowed to culture at 37° C. The opticaldensity of the solution at 600 nm was measured at 1 hour, 2 hours and120 hours duration. The results summarized below confirmed that bothcopper and silver metal coating prepared by this method retarded thegrowth of E. Coli in solution.

TABLE 2 OD at 5 OD at 1 OD at 2 OD at 120 Example minutes hour hourshours Control 0.004 0.006 0.008 1.828 Example 2 0.070 0.080 0.080 0.127Example 6 0.017 0.016 0.021 0.490

Example 19

The silver coating prepared in Example 6 was tested for thermal effectwhen a voltage was applied across the coated substrate using DC powersupply (X-Tech). The temperature of the coating was observed using aFluke Thermal imager. The results are summarized in Table 3 below. Thus,an application of a low voltage to coated filter can heat the filter toas high as 120° C. and offer a mechanism for destroying the microbescolonized or attached to the membrane, renewing its efficacy.

TABLE 3 Voltage Current Temperature of Coating 1.0 volt 0.07 Amp 55° C.1.3 volt 0.10 Amp 72° C. 1.4 volt 0.13 Amp 87° C. 1.5 volt 0.17 Amp 120°C. 

1. A filtration system, comprising: a filter including a poroussubstrate having a metal coating on at least one surface thereof,wherein the metal coating is reduced from a metal salt at ambienttemperature, and exhibits a surface resistivity of less than 1×10¹²Ω/□,and wherein the filter contains through pores extending continuouslythrough the porous substrate and the metal coating.
 2. The filtrationsystem as in claim 1 wherein the porous substrate is wettable by water.3. The filtration system as in claim 2 wherein the porous substrate isselected from cellulose, blends containing cellulose, porous polyamides,porous polyether sulfone, porous polybenzimidazole, porous glass, andporous ceramic.
 4. The filtration system as in claim 1 wherein the metalcoating is bound to the surface of the porous substrate withoutadditives, fillers, or interfacial layers.
 5. The filtration system asin claim 1 wherein the metal coating is selected from copper, silver,cobalt, nickel, tin, zinc, palladium, and combinations thereof.
 6. Thefiltration system as in claim 1 wherein the metal coating exhibits asurface resistivity of less than 1×10⁶Ω/□.
 7. The filtration system asin claim 1 wherein the metal coating exhibits a surface resistivity ofless than 1×10¹Ω/□.
 8. The filtration system as in claim 1 wherein thefilter is effective in reactive electrochemical filtration.
 9. Thefiltration system as in claim 1 wherein the metal coating is reducedfrom a metal salt under a plasma environment.
 10. The filtration systemas in claim 9 wherein the metal salt is selected from a metal hydroxideand a metal oxide.
 11. A method for treating a fluid, comprising:providing a filter including a porous substrate having a metal coatingthat is reduced from a metal hydroxide or a metal oxide at ambienttemperature; and contacting the metal coating of the filter with thefluid to cause an electrochemical interaction between the filter and thefluid.
 12. The method as in claim 11, including recovering a treatedproduct as the filtrate.